It has been known for a long time to use finely ground red phosphorus to give plastics materials a flame retardant finish. Thus, DE-A-1 931 387 describes self-extinguishing, fibreglass-reinforced polyamides which contain red phosphorus as flame retardant. In addition to its freedom from halogen, red phosphorus has further advantages, for example over halogen compounds as flame retardant such as the smaller quantity required and no or significantly reduced impairment of the mechanical and electrical properties of the plastics materials provided therewith.
However, the processing of red phosphorus is more difficult from the safety point of view. Red phosphorus in a very fine distribution is highly inflammable. At elevated temperature in the presence of moisture and atmospheric oxygen it forms highly toxic phosphorus hydride which is spontaneously inflammable in air.
Many attempts have been made to reduce the problems which occur during the processing of finely divided red phosphorus.
The stabilisation of red phosphorus by precipitation of a shell from silicon dioxide is described in DE-A 19 619 701.
Red phosphorus with a phenyl formaldehyde resin shell is known from DE-A 2 625 673.
EP-A 052 217 describes a method for stabilising red phosphorus by encapsulation with melamine resin.
The encapsulation of flame retardants, including red phosphorus, with a urea resorcinol formaldehyde membrane is described in WO 87/00187.
A stabilised powder of red phosphorus in which the particles are coated with a first shell made of aluminium hydroxide and a second shell made of urea melamine phenyl formaldehyde resin is described in EP-A 195 131. In this complex encapsulation process, the shells have to be applied in separate operations.
Although methods for the microencapsulation of red phosphorus and the products obtained by these methods have already been described, a large amount of phosphine is still liberated. Known microencapsulated products also have the drawback that the mechanical strength of the shell is not always adequate. As a result, the shell of a microencapsulated product is damaged by mechanical stresses which may occur in conveyors and metering devices, so the red phosphorus is no longer adequately protected.